The X-ray intensity of knot HST-1, 0.85 ′′ from the nucleus of the radio galaxy M87, has increased by more than a factor of 50 during the last 5 years. The optical increase is similar and our more limited radio data indicate a commensurate activity. We give the primary results of our Chandra X-ray Observatory monitoring program and consider some of the implications of this extreme variability in a relativistic jet. We find that the data support a 'modest beaming synchrotron' model as indicated in our earlier papers. Based on this model, the decay of the X-ray lightcurve appears to be dominated by light travel time across the emitting region of HST-1, rather than synchrotron loss timescales.

We present an analysis of Chandra observations of two high-redshift clusters of galaxies, Cl J1113.1À2615 at z ¼ 0:725 and Cl J0152.7À1357 at z ¼ 0:833. We find Cl J1113.1À2615 to be morphologically relaxed with a temperature of kT ¼ 4:3 þ0:5 À0:4 keV and a mass (within the virial radius) of 4:3 þ0:8 À0:7 Â 10 14 M . Cl J0152.7À1357, by contrast, is resolved into a northern and southern subcluster, each massive and X-rayluminous, in the process of merging. The temperatures of the subclusters are found to be 5:5 þ0:9 À0:8 and 5:2 þ1:1 À0:9keV, respectively, and we estimate their respective masses to be 6:1 þ1:7 À1:5 Â 10 14 and 5:2 þ1:8 À1:4 Â 10 14 M within the virial radii. A dynamical analysis of the system shows that the subclusters are likely to be gravitationally bound. If the subclusters merge, they will form a system with a mass similar to that of the Coma Cluster. Two-dimensional modeling of the X-ray surface brightness reveals excess emission between the subclusters, suggestive, but not conclusive, evidence of a shock front. We make a first attempt at measuring the cluster M-T relation at z % 0:8 and find no evolution in its normalization, supporting the previous assumption of an unevolving M-T relation when constraining cosmological parameters from cluster evolution studies. A comparison of the cluster properties with those of nearby systems also finds little or no evolution in the L-T relation, the gas fraction-T relation, the -T relation, or the metallicity. These results suggest that, in at least some massive clusters, the hot gas was in place, and containing its metals, at z % 0:8 and thus that the clusters were assembled at redshifts significantly higher than z ¼ 0:8, as predicted in low-M models. We also highlight the need to correct for the degradation of the Chandra ACIS low-energy quantum efficiency in high-redshift cluster studies when the low-energy absorption is often assumed to be the Galactic value, rather than measured.

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